Multi-beam communications satellites (e.g., spot beam satellites) are designed such that a given geographic coverage area is serviced by a pattern of beams. With such multi-beam satellites, in order to avoid or minimize inter-beam interference, certain frequency reuse principles must be applied to the bream patterns of the antenna design. One of the primary guidelines for the beam pattern is that a frequency and polarization combination of one beam cannot be “reused” within a certain distance from another beam of the same frequency and polarization combination. The distance between beams is generally specified as the distance between beam centers of two beams of a same color (two beams of the same frequency band and polarization), where the distance is quantified in terms of the radius r of the beams. If the minimum distance requirements are not followed with regard to two such beams, then the beams will cause unacceptable levels of interference between them. The beam pattern design is commonly referred to as a frequency reuse pattern, where each polarization/frequency pair is diagrammatically reflected by a beam color (or pattern in the case of the black and white figures included herein). In typical systems, a reuse of four means that a set of four adjacent beams will have disjoint frequency and polarization assignments such that none of the beams of each set interfere with each other. In other words, adjacent sets of four beams separate the beams sharing a common frequency and polarization such that (even though they are reusing the same frequency and polarization assignments) the beams of one set will not excessively interfere with the respective beams of an adjacent set.
For example, FIG. 1A illustrates a typical 4-beam reuse pattern of a single satellite 110, where, for example, the striped pattern of the cell 101 on the ground reflects a right-hand polarization of a first frequency or frequency band, the dot pattern of the cell 103 reflects a left-hand polarization of the same frequency band as that of 101, the checkered pattern of the cell 105 reflects a right-hand polarization of a second frequency or frequency band, and the brick pattern of the cell 107 reflects a left-hand polarization of the same frequency band as that of the cell 105. In such a four-color reuse pattern, the distance between the beam centers of two beams of the same color are 2√{square root over (3)}*r apart, where r is the center-to-vertex radius of the hexagonal beam. As a further example, FIG. 1B illustrates a typical 3-color reuse pattern, where (similar to the 4-beam reuse pattern of FIG. 1A) each of the ground cell patterns 111, 113, 115 reflects a particular beam frequency/polarization assignment. In such a three-color reuse pattern, the distance between the beam centers of two beams of the same color are 3*r apart, again where r is the center-to-vertex radius of the hexagonal beam. Accordingly, as illustrated by these Figures, each group of four or three particular polarization/frequency beams is geographically arranged such that a beam of a particular polarization/frequency is not adjacent to any beam of the same polarization/frequency (where such beam pairs of a same polarization/frequency are separated by a required minimum distance).
FIG. 1C illustrates typical frequency band and polarization assignments for the beams of FIG. 1A. With reference to FIGS. 1A and 1C, the satellite 110 (via the downlink antennae 110a, 110b, 110c, 110d) transmits the downlink beams A, B, C, D. Each beam A comprises the RHP for the Ka downlink frequency bands 18.3-18.8 GHz (500 MHz of spectrum for each such beam), each beam B comprises the RHP for the Ka downlink frequency bands 19.7-20.2 GHz (500 MHz of spectrum for each such beam), each beam C comprises the LHP for the Ka downlink frequency bands 18.3-18.8 GHz (500 MHz of spectrum for each such beam), and each beam D comprises the LHP for the Ka downlink frequency bands 19.7-20.2 GHz (500 MHz of spectrum for each such beam).
FIG. 1D illustrates a block diagram of a typical configuration for two transmitters of a satellite downlink antenna, configured to transmit one set of the A, B, C, D (or 1, 2, 3, 4) beams of a four-color reuse pattern. With reference to FIG. 1A, each of the beams of the four-color reuse pattern corresponds a respective one of the beams A, B, C, D (as transmitted by a respective transmitter of FIG. 1D). Each of the transmitters comprises an amplifier 131, 151 (e.g., a traveling wave tube amplifier (TWTA)) and a filter 133, 153. For example, the A and B beams are amplified via the TWTA 131 and the C and D beams are amplified by the TWTA 151. The amplified A+B and C+D beams are then fed into the filters 133, 153, respectively. Each filter splits the combined input into two outputs. Each output connects to an antenna feed designed to transmit the A, B, C, or D beams with either right hand or left hand circular polarization. For example, with reference to FIG. 1D, beams of opposite circular polarization—a right-hand polarized A beam and a right-hand polarized B beam via the filter 133, and a left-hand polarized C beam and a left-hand polarized D beam via the filter 153.
Satellite systems are generally designed to maximize capacity by using all of the available spectrum. For example, if 1000 MHz of spectrum (in both polarizations—right-hand polarization (RHP) and left-hand polarization (LHP)) is available for a particular system, the system theoretically has 2000 MHz of available spectrum for each beam group. With reference to the 4-pattern reuse system of FIG. 1A, for example, each beam represented by the pattern 101 may comprise a RHP of the frequency band 18.3-18.8 GHz, each beam represented by the pattern 103 may comprise a LHP of the frequency band 18.3-18.8 GHz, each beam represented by the pattern 105 may comprise a RHP of the frequency band 19.7-20.2 GHz, and each beam represented by the pattern 107 may comprise a LHP of the frequency band 19.7-20.2 GHz. Each beam would thereby comprise 500 MHz of spectrum or bandwidth, for a total available capacity of 2,000 MHz within each 4-beam group. The reuse pattern can be repeated as many times as desired, up to a maximum desired coverage region, as limited by applicable physical constraints, such as total power and mass limits of the overall satellite payload. The total system bandwidth is then the sum of the individual bandwidths of all the beams.
The size of a spot beam is determined primarily by the size of the antenna on the satellite—the larger the antenna, the smaller the spot beam. Further, as would be recognized by one of skill in the art, in order to achieve reasonably acceptable RF performance, the number of beams and the reuse pattern employed will impose certain payload design requirements, such as the number of antennae and the size of each antenna required to implement the desired beam pattern. To cover the eastern half of the continental United States (CONUS), for example, one might design a satellite payload with 50 beams, each of approximately 0.5 degrees diameter, using a three-color reuse pattern. The antennas of such a payload might each be approximately 2.5 m in diameter and three or even four such antennae (e.g., 110a, 110b, 110c, 110d) may be required to achieve desired RF performance. Each beam may be assigned 666 MHz, yielding a total of 33.3 GHz of bandwidth. Accordingly, the desired number of beams, reuse pattern and total capacity will contribute to payload size, weight and power requirements, which in turn will drive up the satellite manufacturing and launch costs.
Moreover, in practice, the distribution of users and associated capacity demand within the cell coverage area is non-uniform, which drives the goal of a satellite system design to provide a corresponding non-uniform distribution of capacity density to satisfy the respective demand. Accordingly, in recent times, some satellite system designs have attempted to solve capacity density requirements by deploying such satellite technologies as steerable beams. FIG. 1E illustrates the four pattern reuse plan of FIG. 1A, where the beams 1, 2, 3, 4 represent the respective cell patterns 101, 103, 105, 107 on the ground, and the beam pattern has been overlaid on a map of the Northeastern United States. As further illustrated in FIG. 1E, in order to provide higher capacity density to the New York/Long Island, Southern Connecticut and Boston areas, certain of the beams have been steered to double the capacity over these regions (e.g., the 3 beam 121 has been moved to the cell 122, the 1 beam 123 has been moved to the cell 124, the 3 beam 125 has been moved to the cell 126, and the 2 beam 127 has been moved to the cell 128). Accordingly, the capacity density has been adjusted to double the spectrum/capacity delivered to the cells 122, 124, 126, 128. This capacity density adjustment, however, has been achieved at the expense of the capacity delivered to the cells 121, 123, 125, 127—as spectrum cannot be provided to these cells without violating the adjacent cell polarization/frequency requirements.
An alternative design may provide for a higher per-beam spectrum allocation. In view of such constraints as satellite size, weight and power, however, such a design would limit the total number of beams available at the higher spectrum allocation. Further, providing for such high capacity beams also significantly increases satellite complexity. Accordingly, with this design, there may not be enough user beams to cover the contiguous United States, and thus the capacity would have to be provided to the higher density population areas at the expense of having no capacity provided to the lower density population areas (e.g., providing user beams over only the Eastern and Western coasts of the United States. Accordingly, again, the desired capacity density allocation is achieved at the expense of being unable to provide capacity to certain geographic regions.
What is needed, therefore, are approaches for a satellite communications system that provides for spot beams of increased capacity density without sacrificing capacity in adjacent beams and without overly increasing satellite size, weight, power and complexity constraints.
A significant contributor to the system cost of a satellite system is the launch cost. Recently, new launch services providers have become viable that are capable of launching smaller conventional satellites for competitive costs. There is now a market for launching large satellites as described above as well as multiple smaller satellites.